Re-buid my iMac

I have my iMac for 4 years and have updated from Leopard --> Snow Leopard --> Lion --> Mountain Lion. 

I , finally, feel that he is not that responsive ...... so I decide to re-install the Mountain Lion from sketch.... as I  hear that the re-install of the OS would result to be massively fast.

Will tell the result after that ........

And write down some tips or experiences of this operation.

First.... it will be how to make the bootable USB drive. :D